1. Field of the Invention
The present invention relates to an image display device and a transmission signal control method to be used in same and more particularly to the image display device and the transmission signal control method that can be suitably used when transmission wirings for data signal based on a video signal become complicated due to configurations of a large-sized and high-definition image display device.
2. Description of the Related Art
In an image display device such as a liquid crystal display device, there are mounted a driver IC to drive a display panel, a timing controller to output a control signal obtained by performing timing control and/or rearrangement process to an inputted video signal, to the driver IC, a power supply circuit to supply power to these ICs, and the like. Here, transmission of data signal to be inputted to the driver IC used to drive the display panel is studied. The driver IC and timing controller IC are electrically connected to each other through data signal transmission wirings. As a method for transmitting a data signal through the data signal transmission wirings, there are provided a parallel transmission method such as a CMOS transmission method and a differential signal transmission method such as a RSDS (Reduced Swing Differential Signaling) transmission method, and a mini-LVDS (Low Voltage Differential Signaling) transmission method.
However, the above methods for transmitting the data signal are one of reasons for heightening EMI (Electro-Magnetic Interference) emission levels in an image display device. In particular, in the case of using the CMOS transmission method, in the data signal transmission wirings each having the number corresponding to a gray level of a video signal to be inputted, the swing of a data signal occurs to cause a change of signal waveform within a voltage range (for example, 3.3V to 0V) between a source voltage and a ground level, which becomes a reason for the occurrence of a non-negligible amount of the EMI emissions. To reduce the EMI emissions, a transmission method using an invert signal is available. According to this transmission method, in the data signal transmission wirings each having the number corresponding to the gray level, all video signals are not swung, but, when the gray level of a present data signal is compared with a gray level of the data signal occurred before being transmitted and if an amount of change in gray levels is large, an invert signal is changed and polarities of all the signal are inverted. Even in the case of using the above method, in many cases, an image display device, a large-sized and high-definition type image display device in particular, is easily influenced by a gray level of a video signal to be inputted, arrangement of data signal transmission wirings, ground loop state of the image display device, thus resulting in suffering from high EMI emission levels, which further requires countermeasures.
In this case, a polarity of an invert signal is judged and determined for setting by comparing a change in polarity of a video signal with a previous gray level and, therefore, even in the case of a high level (H) or of a low level (L), the image display device operates normally and, as a result, the polarity of the invert signal at its initial stage is in an unstable state. Moreover, with the aim of avoiding a change in current to be consumed by the image display device, in many cases, all the polarities of the invert signal are fixedly set to be the same. In this state, a magnetic field is interfered and, in part out of current loops of an entire image display device, there probably occur many areas where magnetic fields are strengthened each other. For this reason, there is a fear that a non-negligible amount of the EMI emission occurs.
As a related art of this type, a driving circuit of a liquid crystal display device is disclosed in Patent Reference No. 1 (Japanese Patent Application Laid-open No. 2001-166740). In the driving circuit, as shown in FIG. 19, in every output port out of four ports, when the number of data signals whose polarities change exceeds the majority out of all outputs to bus lines, inverts the polarities of all data signals and outputs data signals BUS-A1-A24, BUS-B1 to B24, BUS-C1 to C24, BUS-D1 to D24 to bus lines from each output port. Moreover, as shown in FIG. 19, a controller 2 outputs, from every output port out of four output ports, polarity inverted signals INV-A to D each showing that the polarity of a data signal to be outputted has been inverted, to bus lines and, therefore, it is possible that the number of data signals whose polarities change can be reduced to a half or less of the number of data signals to be transferred.
However, the above related arts have following problems. That is, the driving circuit disclosed in the Patent Reference 1 is configured to reduce the number of data signals to be transferred through bus lines whose polarities change, but is not so configured that electromagnetic fields generated by currents cancel one another out. Therefore, the direction of a current flowing through the entire liquid crystal device cannot be controlled, thus it is made impossible to sufficiently reduce EMI emissions.